average of cells from user selected range of worksheets

Hi, I am going to give an example the best I can.

I have Sheet1-Sheet10. Cell A10 on all sheets contains a number. I have a seprate worksheet I refer to as "averages" I have a macro like this thus far that works so far:

Sub average()
Dim sh  ' a variable to hold the sheets
Dim ct  ' a variable for counting sheets (although could use worksheets.count to do this
Dim av  ' a variable used for the average
For Each sh In ActiveWorkbook.Worksheets
   If sh.Name <> ActiveWorkbook.Name Then
       av = av + sh.Range("A10").Value
       ct = ct + 1
   End If
Next
ActiveSheet.Range("A10").Value = av / ct
End Sub

I would like to be able to create an input box or drop down box to select any worksheets at random and only get back the average of those selected. An example would be I would tell excel somehow to look at Sheet2-Sheet7 and give me the average of A10. Next time I would ask for the average from Sheet1-Sheet8. Thanks for looking!